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Diffstat (limited to 'Documentation/drivers/dt_entries.md')
| -rw-r--r-- | Documentation/drivers/dt_entries.md | 32 |
1 files changed, 30 insertions, 2 deletions
diff --git a/Documentation/drivers/dt_entries.md b/Documentation/drivers/dt_entries.md index 156a672a5920..7726b4586700 100644 --- a/Documentation/drivers/dt_entries.md +++ b/Documentation/drivers/dt_entries.md @@ -9,6 +9,7 @@ device pci 15.0 on register "hid" = ""ELAN0000"" register "desc" = ""ELAN Touchpad"" register "irq" = "ACPI_IRQ_WAKE_LEVEL_LOW(GPP_A21_IRQ)" + register "detect" = "1" register "wake" = "GPE0_DW0_21" device i2c 15 on end end @@ -140,6 +141,31 @@ find the names in your SoC's header file. The ACPI_* macros are defined in Using a GPIO as an IRQ requires that it is configured in coreboot correctly. This is often done in a mainboard-specific file named ``gpio.c``. +### detect + +The next register is: + +``` + register "detect" = "1" +``` + +This flag tells the I2C driver that it should attempt to detect the presence of +the device (using an I2C zero-byte write), and only generate a SSDT entry if the +device is actually present. This alleviates the OS from having to determine if +a device is present or not (ChromeOS/Linux) and prevents resource conflict/ +driver issues (Windows). + +Currently, the detect feature works and is hooked up for all I2C touchpads, +and should be used any time a board has multiple touchpad options. +I2C audio devices should also work without issue. + +Touchscreens can use this feature as well, but special care is needed to +implement the proper power sequencing for the device to be detected. Generally, +this means driving the enable GPIO high and holding the reset GPIO low in early +GPIO init (bootblock/romstage), then releasing reset in ramstage. While no +boards in the tree currently implement this, it has been used in downstream +forks without issue for some time now. + ### wake The last register is: @@ -206,5 +232,7 @@ Name (_CRS, ResourceTemplate () // _CRS: Current Resource Settings ## Notes - - **All devices in devicetrees end up in the SSDT table, and are generated in - coreboot's ramstage** + - **All device driver entries in devicetrees end up in the SSDT table, and are + generated in coreboot's ramstage** + (The lone exception to this rule is i2c touchpads with the 'detect' flag set; + in this case, devices not present will not be added to the SSDT) |